ARTICLE pubs.acs.org/IECR
A Comparative Study of Turbulent Premixed Flames Propagating Past Repeated Obstacles A. R. Masri,† A. AlHarbi,† S. Meares,*,† and S. S. Ibrahim‡ † ‡
School of Aeronautical, Mechanical and Mechatronic Engineering, The University of Sydney, NSW, 2006 Australia Department of Aeronautical and Automotive Engineering, Loughborough University, Loughborough LE11 3TU, U.K. ABSTRACT: This paper presents a comparison of the overpressures and pressure gradients obtained in turbulent premixed flames of liquified petroleum gas (LPG), compressed natural gas (CNG), and hydrogen propagating past solid obstacles. The fuel-air mixture is initially at rest and is ignited at the base of a vented chamber. For each fuel a total of eighteen configurations are studied here with various permutations of three baffles plates and an obstacle with a square cross section providing a blockage ratio of either 0.24 or 0.5. High speed laser induced fluorescence from OH (LIF-OH) imaging is also performed at a repetition rate of 5kHz providing new measurements of the evolution of the flame front along the middle section of the chamber. It is found that for all fuels the peak pressure as well as the rate change of pressure increases not only with increasing blockage ratio induced by the increasing blockage ratio, but also with decreasing separation between successive baffles. The peak pressure and its rate of change in hydrogen flames are over an order of magnitude higher than those obtained in LPG and CNG fuels. High speed images of LIF-OH show that the degree of wrinkling and contortion in the flame front also increases significantly with increasing blockages. It is evident from the LIF-OH images that the flame front relaminarises as the separation between successive obstacles increase hence explaining the pressure decrease with increasing separation.
1. INTRODUCTION Turbulent premixed propagating flames interacting with one or more solid obstacles are relevant and interesting but still pose a significant challenge to modelers. The relevance is because these scenarios are representative of combustion in many practical devices such as internal combustion engines as well as of industrial explosions that lead to deflagrations (and potentially transition to detonations although this latter phenomenon is not studied here). The challenge arises due to the complexity of the flame front’s interaction with the turbulence generated ahead of it by the expanding gas. Such interactions impact strongly on the resulting overpressure, the rate of pressure rise, the burning rate of the gases, and the geometry of the accelerating flame front. This paper sheds more light on these processes through detailed measurements in a controlled deflagration chamber. Earlier work in laboratory-scale chambers1 3 as well as in large scale installations4 15 has already investigated numerous aspects of the impact of obstacles on deflagrating flames. Laboratory studies have explored a range of geometries in enclosed cylindrical vessels with or without obstacles,4,5 cylindrical vessels with turbulence inducing rings,6,7 or circular plate obstructions8 chambers with a rectangular cross-section using a single plate as an internal baffle9 or with a square cross-section and multiple baffles lining the top and bottom walls of the chamber.10 Alexiou et al.11 studied the effects of positioning the vent on the side or at the end of a cylindrical vessel. Moen et al.12 used streak photography to measure the flame acceleration between two circular plates separated by varying distances where one disk is lined with a spiral tube of a given pitch and diameter acting as an obstacle. Patel et al.13 studied deflagrations in a square crosssection chamber using flat plates as obstacles placed repeatedly at various distances along the chamber’s length. The current r 2011 American Chemical Society
authors have also made a contribution by reporting the effects of obstacle geometry, blockage ratio, and obstacle position on the overpressure and flame structure in a chamber of square cross section using LPG as a fuel.14 16 In all these geometries, the chamber’s length, L, in the direction of the propagating flame is large compared to its diameter, D or the width of the its base, W. Ratios of L/D (or L/W) in the above references range from about 27 to 3210. However, Lohrer et al.17 recently reported measurements of velocity and turbulence in pipes with L/D ratios changing from 4 to 143, with and without obstructions operating at different initial pressures. It is noted that transition to detonation tends to occur at high ratio of L/D, and this transition is enhanced by turbulence induced by the presence of obstacles However, this domain of operation remains outside the scope of paper which focuses on deflagrations. Park et al.,18 studying deflagrations in chambers with low L/D ratio of 0.235, reported that flame displacement speeds were not significantly affected by the obstacle geometry or blockage ratio. While this is different from the trends observed at high L/D, it may be due to the fact that turbulence levels remain low and not well developed at these low values of L/D. Capabilities to compute the structure of turbulent premixed flames are evolving and advanced methods such as large eddy simulations (LES) are now used to predict the evolution of the reaction zones in time and three-dimensional space.19 22 Special Issue: Russo Issue Received: August 29, 2011 Accepted: December 15, 2011 Revised: December 12, 2011 Published: December 15, 2011 7690
dx.doi.org/10.1021/ie201928g | Ind. Eng. Chem. Res. 2012, 51, 7690–7703
Industrial & Engineering Chemistry Research
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A natural limitation of LES methods is that they require modeling for processes that occur below a certain cutoff scale as applies to chemical reaction. This is referred to as subgrid scale modeling (SGS). A range of SGS modeling approaches are available with varying degrees of complexity and Di Sarli et al.23 have presented a validation of some of these SGS models for premixed flame propagation. Flamelet based methods24 26 are limited to thin reaction zones and include a number of variations such as (i) flame generated manifolds (FGM) tabulated in terms of mixture fraction, reaction progress variable as well as other parameters such as a measure of strain,27,28 (ii) flame surface density (FSD) models where a transport for the FSD is solved,29 33 and (iii) thickened flamelets.34,35 SGS modeling approaches that are seen as alternatives to flamelets include the filtered density functions,36 38 the conditional moment closure,39 41 and the linear eddy model.42,43 Validation of these approaches is an essential step for further development and this requires extensive and reliable data collected in relevant burners and for a variety of fuels and flame configurations. The interactions between measurements and LES calculations in transient propagating flames have already proven to be extremely useful in advancing modeling capabilities.31,32,44,45 The provision of additional measurements that show the time-evolution of the propagating flame fronts would also be extremely useful for comparison with the LES calculations. Experimental capabilities to provide such information have only recently become available with the advent of high-speed lasers, cameras, and intensifiers. This paper reports comparative measurements of pressuretime traces and rates of pressure increases in deflagrating flames of hydrogen, compressed natural gas (CNG), and liquefied petroleum gas (LPG). The transient, turbulent flame fronts propagate past a sequence of solid obstacles covering a range of blockage ratios. Hydrogen is of particular relevance here considering its increasing usage as a clean fuel and energy carrier that can be derived from gasification processes of either fossil fuels or biomass and used in fuel cells. With this increasing usage, the risk of hydrogen leaks and potential explosion becomes serious considering its wide flammability limits and low ignition energy. The paper also reports the first high-speed planar images of laser induced fluorescence from OH (HS-LIF-OH) which is used here for convenience as a marker of the reaction front. The images are collected at a repetition rate of 5kHz and this is adequate to resolve the time sequences of the propagating flames.
2. EXPERIMENTAL SECTION 2.1. The Combustion Chamber. The same combustion chamber used in earlier studies and shown schematically in Figure 1a is adopted here for convenience. It is square in crosssection with internal dimensions of length, L = 250 mm and side, W = 50 mm producing an overall volume of 0.625 liters and a ratio L/W = 5. The test section is contained within two layers of Perspex: An inner assembly enabling various geometrical modifications, comprising several Perspex spacers to separate the grids and obstructions, and an outer prism constructed from 20 mm thick Perspex that provides structure to the inner layer by encompassing the entire assembly. Up to three baffle plates (also interchangeably referred to as grids or obstacles) may be placed in the chamber at various distances from the ignition source at the base of the chamber. These may be located between the inner perspex spacers at any one of three locations being 19 mm
Figure 1. (a) Vertical cross section of the combustion chamber showing ignition lens, baffles, obstacles, venting flap, and silica fused windows; (b) the removable baffles.
(Grid 1), 49 mm (Grid 2), and 79 mm (Grid 3) from the base (these position dimensions refer to the base of the baffle thickness). The baffle plates, a schematic of which is also shown in Figure 1b consist of five strips, 4 mm wide, evenly separated by six gaps, 5 mm wide, thus creating an overall blockage ratio of 0.4. Downstream of the baffle plates, a further obstruction with a square cross section may be placed such that its lower surface is maintained at 96 mm from the base plate. Two solid obstructions are used, a small one with a cross section of 12 12 mm2 and a large one with a 25 25 mm2 cross section. The blockage ratios of these square obstructions are 0.24 and 0.5 respectively. Three different fuels are used: hydrogen, liquified petroleum gas, LPG (95% C3H8, 4% C4H10 and 1% C5+ hydrocarbons by vol.), 7691
dx.doi.org/10.1021/ie201928g |Ind. Eng. Chem. Res. 2012, 51, 7690–7703
Industrial & Engineering Chemistry Research
ARTICLE
Figure 2. (a) Schematic of the high speed LIF-OH imaging equipment (b) the three imaging tiers used to capture the maximum viewable height.
and compressed natural gas, CNG (88.8% CH4, 7.8% C2H4, 1.9% CO2 and 1.2% N2 with the remaining 0.3% being a mixture of propane, propene, butane and pentane). The fuel air mixture enters the atmospheric pressure chamber through a nonreturn valve, seen at the base of Figure 1a. The operating conditions in preparation for each combustion event are optimized to ensure repeatability and the process is described here as follows: Before each ignition event, the fuel air mixture is injected for 10 s at a flow rate of 26.21 L/m (LPG). This provides 7 multiples of the volume of the chamber to purge gases from the previous cycle. The flow is then stopped and the gases within the
chamber are allowed to settle for 10 (LPG), 15 (CNG), or 5 (H2) seconds before the stagnant mixture is ignited by focusing the infrared output from a Nd:YAG laser 2 mm above the base. Laser timing is controlled by the Q-switch of the Nd:YAG and this marks the start of each experiment, or time zero. A hinged flap on top of the chamber contains the mixture during fill time prior to ignition. This flap rises 1 s before ignition and is maintained as such through the combustion process to allow venting. Pressure is recorded at 25 kHz using two Keller type PR21-SR piezoelectric pressure transducers with a range of 0 1 bar and a total error
